Background and aims: The coronavirus disease 2019 (COVID-19) pandemic is severely threatening and challenging public health worldwide. Epidemiological studies focused on the influence of outdoor air pollution (AP) on COVID-19 risk have produced inconsistent conclusions. We aimed to quantitatively explore this association using a meta-analysis. Methods: We searched for studies related to outdoor AP and COVID-19 risk in the Embase, PubMed, and Web of Science databases. No language restriction was utilized. The search date entries were up to August 13, 2021. Pooled estimates and 95% confidence intervals (CIs) were obtained with random-/fixed-effects models. PROSPERO registration number: CRD42021244656. Results: A total of 35 articles were eligible for the meta-analysis. For long-term exposure to AP, COVID-19 incidence was positively associated with 1 μg/m increase in nitrogen dioxide (NO; effect size = 1.042, 95% CI 1.017–1.068), particulate matter with diameter <2.5 μm (PM; effect size = 1.056, 95% CI 1.039–1.072), and sulfur dioxide (SO; effect size = 1.071, 95% CI 1.002–1.145). The COVID-19 mortality was positively associated with 1 μg/m increase in nitrogen dioxide (NO; effect size = 1.034, 95% CI 1.006–1.063), PM (effect size = 1.047, 95% CI 1.025–1.1071). For short-term exposure to air pollutants, COVID-19 incidence was positively associated with 1 unit increase in air quality index (effect size = 1.001, 95% CI 1.001–1.002), 1 μg/m increase NO (effect size = 1.014, 95% CI 1.011–1.016), particulate matter with diameter <10 μm (PM; effect size = 1.005, 95% CI 1.003–1.008), PM (effect size = 1.003, 95% CI 1.002–1.004), and SO (effect size = 1.015, 95% CI 1.007–1.023). Conclusions: Outdoor air pollutants are detrimental factors to COVID-19 outcomes. Measurements beneficial to reducing pollutant levels might also reduce the burden of the pandemic.